Electrically-variable transmissions (EVTs) typically have an input member connected to an engine, as well as one or two motor/generators that are connected to different members of planetary gear sets to enable one or more electrically-variable modes of operation, fixed speed ratio modes, and an electric-only (battery powered) mode. An “electrically-variable” mode is an operating mode in which the speed ratio between the transmission input and output members is determined by the speed of one of the motor/generators.
EVTs may improve vehicle fuel economy in a variety of ways. For instance, the engine may be turned off at idle, during periods of deceleration and braking, and during periods of relatively low speed or light vehicle load operation to thereby eliminate efficiency losses incurred due to engine drag. Captured braking energy via regenerative braking, or energy stored by one of the motors acting as a generator during periods when the engine is operating, is utilized during these “engine off” periods to extend the period or duration during which the engine is off, to supplement engine torque or power, to operate the vehicle at a lower engine speed, and/or to supplement accessory power supplies. Transient demand for engine torque or power is supplemented by the motor/generators during “engine on” periods, allowing for downsizing of the engine without reducing apparent vehicle performance. Additionally, the motor/generators are very efficient in accessory power generation, and electric power from the battery serves as an available torque reserve allowing operation at a relatively low transmission speed ratio.
Electrically-variable modes may be classified as input-split, output-split, compound-split, or series modes. Input-split modes gear one motor/generator such that its speed varies in direct proportion to the transmission output, and the other motor/generator such that its speed is a linear combination of the input and output member speeds. Output-split modes have one motor/generator geared such that its speed varies in direct proportion to the transmission input member, and have the other motor/generator geared such that its speed is a linear combination of the input member and the output member speeds. A compound-split mode has both motor/generators geared such that their speeds are linear combinations of the input and output member speeds, but neither is in direct proportion to either the speed of the input member or the speed of the output member. A series mode has one motor/generator geared such that its speed varies in direct proportion to the speed of the transmission input member, and another motor/generator geared such that its speed varies in direct proportion to the speed of the transmission output member. There is no direct mechanical power transmission path between the input and output members when operating in series mode, and therefore all power must be transmitted electrically.
A series propulsion system is a system in which energy follows a path from an engine to an electric storage device, and then to an electrical motor/generator which applies power to rotate the drive members. In other words, there is no direct mechanical connection between the engine and the drive members in a series propulsion system, unlike parallel propulsion systems
Existing EVT architectures are predominantly single mode input-split or two-mode. These architectures are not optimized for a plug-in hybrid application with a small engine and large battery where a large portion of operation is with engine off. For example, the single mode input-split requires that the output motor/generator be designed for the full vehicle speed range. The motor/generator thus may need to be designed to accommodate the high motor top speed in order to provide the desired motor strength and/or provide reasonable losses at top speed. A high system voltage (e.g., 600V) may also be required in order to make this system feasible. Motor/generator and transmission spin losses will tend to be relatively high in this type of system at high vehicle speeds, reducing highway fuel economy. A two-speed transmission may be used on the output motor to reduce the speed range of the motor/generator at the cost of increased mechanical complexity. A second source of increased losses when engine is off is that the input generator typically spins at several times output speed but is not usable for vehicle propulsion. Also, in a single mode input-split, a large percentage of the engine power flows through the electrical path at high engine speeds.
Two-mode architectures are not optimized for operating at high speed with engine off. The input motor (motor/generator A) may spin at several times the output speed with the engine off, but is not usable for propulsion, contributing to high spin losses and limiting the top speed of the vehicle with engine off. Powertrains with two-mode transmissions may also have a fixed gear as the synchronous point between first and the second electrically-variable modes. This requires that the engine speed must flare to the fixed gear synchronous speed when transitioning the engine from on to off or from off to on at high vehicle speeds. Also, the motor/generators are typically suited to be roughly equal in size, which makes it difficult to use a large amount of available battery power for responding to driver transient demand unless both motors are oversized.